In recent years, several new non-destructive analytical techniques have been developed for investigating the internal properties of materials. By utilizing a technique which has been variously referred to as "computer tomography", "computerized axial tomography", "transaxial tomography" and "reconstruction from projections", it is possible to generate a picture of an object along a thin cross-sectional slice of the body. At present, there are five general areas which utilize tomographic techniques for generating cross-sectional pictures of objects, and these are transmission tomography, emission tomography, ultrasound tomography, electrical impedance tomography, and nuclear magnetic resonance tomography. Each of these areas will be summarized below.
Much of the early work in the field of image construction utilizing tomographic techniques centered on the use of X-rays and other narrow beam penetrating radiation, for example gamma rays. These transmission techniques have found major application in the medical diagnostic field although other analytical applications have been described. In the X-ray tomography technique, a two dimensional cross-sectional image is reconstructed by taking a large number of transmission measurements through the slice of interest using an X-ray source-detector assembly. The assembly is rotated uniformly about the body as the measurements are taken, and by restricting the measurements to parts of the body contained only in the slice of interest, information from other parts of the body is automatically excluded from the data. The measurements are usually carried out at a single frequency, and the quantity measured and displayed is the X-ray absorption at each volume element (or pixel) within the slice of interest. In the normal arrangement for medical application, the patient remains stationary and the X-ray source-detector assembly is translated and rotated around the patient. However, in other possible applications, the object may be moved relative to the X-ray source-detector assembly. In more recent machines, the X-rays are formed into a fan of pencil beams which encompasses the section of interest so that movement of the X-ray source-detector assembly and the object relative to each other is significantly reduced. Although modern X-ray tomography machines are extremely fast in the data collection process, typically taking between five and ten seconds for the whole operation, there is nevertheless the acknowledged radiation health hazard due to the presence of X-rays. In addition, X-ray tomography equipment is extremely costly from both the manufacturing and operation/maintainence standpoints. In view of this, research into the development of lower cost X-ray tomography equipment with improved radiation shielding and lower exposure times is currently underway, but the radiation hazard to the patient does nevertheless detract from the use of this technique.
Emission tomography differs somewhat from transmission tomography in that, instead of beaming penetrating radiation through a selected slice of the body and measuring the amount coming out of the other side, as in X-ray tomography, penetrating radiation emitted from special radioactive chemicals taken into the body is measured and used to generate an image of the area through which the emission beam passes. Like X-ray tomography, emission tomography suffers from the same health hazard problem arising from the radioactive emissions. Further disadvantages arise from the fact that the radioactive organic compounds first must be synthesized, usually requiring the use of a cyclotron which is an extremely expensive process, and then the compounds, when so formed, must be used almost immediately because of their relatively short halflife.
In the ultrasound tomography technique, high-frequency ultrasonic pulses are transmitted into the body and, using tomographic techniques, pictures of the shape of tissues are reconstructed from reflected pulses (echo data). As the traveling ultrasonic pulses encounter changes in densities in the body, part of the pulse is reflected back to a detector which is usually pressed against the body. The other part of the pulse not encountering a change in density travels on through the body. Thus, ultrasound tomography differs from transmission and emission tomography in that it is a pulse reflection technique, and in most instances requires the detector to be in physical contact with the body under investigation. An advantage of the use of high-frequency ultrasonic pulses is that there is no known health hazard to the patient. However, this technique suffers from several disadvantages, the main ones being that the echo data often contains considerable noise, and the pulse detector must be in good skin contact on the surface of the body to ensure that the reflected pulses are detected.
The technique of electrical impedance computed tomography does not employ high-frequency ultrasonic waves or penetrating radiation such as X-rays, but instead uses weak electrical currents to map out the electrical properties of the material, such as human tissue. While this technique has the advantage of reduced health hazard, it suffers from the same disadvantage as the ultrasonic tomographic technique in that the detectors must be placed in good contact with the skin of the patient. Thus, it is essential to establish good electrical contact between the skin of the patient and the electrode detector system, otherwise it is impossible to detect with any degree of accuracy the weak electrical currents which are used. In this event, the resulting reconstructed images have very poor resolution.
A recent development in the tomography area is nuclear magnetic resonance (nmr) tomography, also known as zeugmatography. This technique makes use of the fact that nuclei, typically individual protons or hydrogen nuclei, have a small nuclear moment and an associated spin angular momentum. The combined effect of the magnetic moment and spin gives rise to precession on the nuclei about the direction of an applied magnetic field similar to the way in which a spinning top precesses when perturbed from the upright position. In the nmr technique, a magnetic gradient is applied to the sample, and the nuclei tend to polarize or align themselves along the direction of the magnetic field giving rise to a bulk magnetization of the sample. A perturbing pulse or pulses is applied to the sample to perturb the magnetization through, for example 90.degree., and repolarization occurs according to the spin-lattice relaxation time which is characteristic of the electronic environment of the nuclei. The frequency of precession, known as the Larmor frequency, is typically in the range of 10 to 100 MHz, and this requires the application of relatively high strength magnetic fields arranging from about 0.235 to 2.35 Teslar (corresponding to about 2.35 to 23.5 kilogauss). By applying the magnetic field gradient through a cross-section of the body, it is possible to obtain the spin-lattice relaxation times across the cross-section and to use these to create a picture reflective of the variation of proton density over the cross-section. Although the Larmor technique appears to have good potential in the diagostic field, it is not yet known whether ill effects arise as a result of being subjected to the relatively high magnetic field strengths involved. A major disadvantage of the technique is that presently available devices are incapable of penetrating more than about one or two inches into the tissue.
A recent approach which has been taken in the nondestructive testing area is to investigate the dielectric constant and conductivity properties of materials. In a recent article appearing in "Electronics International", published Jan. 23, 1980, an instrument referred to as a "mammo-scanner" is described, which allegedly aids in the early detection of breast cancer. The mammo-scanner appears to be the subject of U.S. Pat. No. 4,144,877 to Frei et al, issued Mar. 20, 1979, and operates by measuring the variations of dielectric constant and conductivity of the breast tissue. This technique is essentially the same as electrical impedance computer tomography discussed earlier, in that the detector must be contacted with the tissue in order to obtain useful output data. The device shown in the Frei patent is constructed in the form of a hand glove which permits the examiner to conduct a palpation procedure over the entire surface of the breast tissue. The variations in dielectric constant are displayed, for example, on a gray scale, and any large fluctuations over the surface of the breast tissue indicate that further investigation for possible cancerous tissue may be required.
U.S. patent application Ser. No. 28,452, assigned to Georgetown University, the disclosure of which is specifically incorporated herein by reference, relates to identification of materials using their dielectric constant and conductivity properties. Identification is achieved by bringing the entire sample to be identified into the influence of a field and measuring the dielectric and conductance properties of the material as a whole over a range of pre-selected frequencies. It is possible to directly identify the materials under investigation using this technique, typically explosives and drugs, but some difficulties arise when these materials form only a part of a larger object possibly composed of many component materials which is brought into the influence of the field. This situation arises, for example, when a small volume of an explosive material or a drug is concealed in a large package such as a mail parcel or piece of hand luggage. The situation can also arise in the medical field where a small mass of cancerous tissue is present in a large area of healthy tissue. In these instances, it is difficult to accurately identify the concealed small volume sample using the technique of the above United States patent application, which means that the technique is limited essentially to an overall identification of the entire volume of material brought within the influence of the field.
U.S. patent application Ser. No. 871,099, now U.S. Pat. No. 4,263,551, assigned to Georgetown University, the disclosure of which is specifically incorporated herein by reference, relates to identification of conductive materials by measuring the true resistive component of the impedance change which occurs when the material is brought within the influence of a magnetic field generated by a stable coil system. When the true resistive component is divided by the respective applied frequency, a value is obtained which varies with frequency and peaks at a single peak frequency. At the peak frequency, the value of the true resistive component divided by the frequency is proportional to the resistivity of the material divided by its cross-sectional area. Again, it is difficult to accurately identify a concealed small volume sample of conductive material using this technique.